A plane not visible in Viewport appears in render
First press the button marked red, then enable the option in marked yellow. It should make the plane appear in the viewport and you can remove it. Don't ask me why it's not enabled by default. This is the method to hide objects in 3d View but not for the actual rendering. It's meant to work across linked copies in different scenes or separate files.
Also see: https://docs.blender.org/manual/en/latest/editors/outliner.html#restriction-columns